This invention relates to external cavity lasers of the kind which employ a retroreflector and an intra-cavity angle-tuned filter, and is applicable to both tunable lasers and fixed-wavelength lasers. The invention is also applicable to a method of making such fixed-wavelength lasers.
It is well known to add an external cavity to a semiconductor laser to increase intensity at a given wavelength. It is also known to use a fiber grating in the external cavity to select the wavelength at which the laser emits. If only one output wavelength is desired, a simple grating may suffice. If more than one wavelength is required, however, it is known to use a bulk diffraction grating and tilt it relative to the optical axis of the laser to select different operating wavelengths. It is also known for an external cavity laser that is tunable to different wavelengths to have a mirror and an angle-tuned filter forming the external cavity. A disadvantage of the bulk grating kind of external cavity laser is that it tends to be rather bulky. Although angle-tuned filter external cavity lasers are less bulky, a disadvantage they share with bulk grating designs is that the mechanical system for tilting the filter, like the grating, must be very precise, which tends to increase manufacturing costs by an unacceptable amount. Moreover, the precision tuning mechanism is very susceptible to changes in temperature, mechanical vibration, and so on.
An improved design of angle-tuned external cavity laser was disclosed by Zorabedian in a paper entitled xe2x80x9cInterference Filter Tuned, Alignment Stabilized, Semiconductor External Cavity Laserxe2x80x9d, Optics Letters, Vol. 13, No. 10, pp. 826-828, October 1988. The angle-tuned filter is positioned between the laser and a retroreflector formed by a converging lens and a mirror, the latter extending through the focal point of the lens. Because the reflection is at the focal length of the lens, this so-called xe2x80x9cdegenerate cavityxe2x80x9d reduces the effect of the orientation of the mirror upon the accuracy with which the light can be returned to the laser and improves performance.
Although Zorabedian""s design reduces the requirement for the precise positioning of the mirror, cost savings and reduced losses are not fully realised because the number of discrete components is increased, which tends to complicate assembly and increase the cost of manufacture.
Hence, known designs are still expensive to make to achieve the necessary precision and ensure that losses are acceptable.
The present invention seeks to eliminate or at least mitigate the disadvantages of such known external cavity lasers.
According to one aspect of the present invention, there is provided an external cavity laser comprising light source means with internal optical gain for providing substantially collimated light, an angle-tuned filter extending across an optical axis of the light source and a retroreflector positioned so as to receive said light from the light source via the angle-tuned filter and reflect the light via the angle-tuned filter back to the light source, wherein the retroreflector comprises a unitary element.
Preferably, the retroreflector comprises a quarter-pitch graded-index (GRIN) lens having a proximal end surface oriented towards the laser and a distal end surface opposite thereto, and a mirror provided on said distal end surface.
The mirror may comprise a coating upon the distal end surface or a separate component bonded to it by suitable optical adhesive.
The angle-tuned filter may comprise a discrete filter separate from the GRIN lens, or bonded to the GRIN lens. Alternatively, the filter may be coated upon the proximal end surface using thin film filter techniques. In the latter case, the tilting mechanism will be arranged to rotate the GRIN lens and with it the angle-tuned filter so as to adjust the tilt angle. The tilt axis around which the GRIN lens is rotated need not intersect the optical axis so long as the filter and the retroreflector remain in the path taken by the light beam.
In preferred embodiments, the angle-tuned filter and the mirror are provided on the proximal and distal end surfaces, respectively, of a GRIN lens, each preferably as a coating, and the tilting mechanism tilts the GRIN lens so as to adjust the angle of the filter relative to the optical axis of the laser.
The angle of inclination of the angle-tuned filter relative to the optical axis, in use, may be fixed or adjustable. In the latter case, therefore, the external cavity laser will further comprise means for adjusting the angle of inclination of the angle-tuned filter.
In the case of a fixed-wavelength external cavity laser, the angle-tuned filter will be adjustable during manufacture using separate adjusting means, which is not part of the external cavity laser, then fixed at a desired tilt angle.
According to a second aspect of the invention, therefore, there is provided a method of making an external cavity laser comprising the steps of assembling a light source means with internal optical gain for supplying substantially collimated light, an angle-tuned filter and a retroreflector with the angle-tuned filter extending across an optical axis of the light source, the retroreflector being positioned so as to receive light from the light source via the angle-tuned filter and reflect the light to pass via the angle-tuned filter back to the light source, adjusting the angle of the angle-tuned filter relative to the optical axis to a predetermined angle, and fixing the angle-tuned filter at that angle.
In embodiments of this second aspect of the invention, the angle-tuned filter may be provided on a GRIN lens and the adjustment of the angle of the angle-tuned filter effected by rotating the GRIN lens relative to the optical axis.